Update (July 5, 2026): FHV-1 Dendritic Keratitis — Modified-Live Vaccine Strain Isolated From a Corneal Ulcer and the Post-Vaccination Corticosteroid Caution

Bottom line

• A 2025 JFMS report isolated the F2 modified-live vaccine (MLV) strain of FHV-1 from a cat's dendritic corneal ulcer that appeared 17 days after a first Purevax RCP vaccination — direct evidence the vaccine strain can replicate in the corneal epithelium and form a lesion.[1]
• It was one of four dendritic-ulcer cats; whole-genome sequencing matched it to commercial F2 strains via unique variants in ORF28 and ORF44. The other three isolates were field strains.[1]
• Three of the four cats had a corticosteroid history; the authors conclude corticosteroids are not recommended immediately after MLV vaccination in cats without prior allergy.[1]
• This reinforces a long-standing rule: corticosteroids trigger FHV-1 recrudescence (~70% of latent cats shed virus after steroids) and are hazardous in fluorescein-positive herpetic keratitis.[2]
• Manage ulcerated herpetic keratitis with antivirals (oral famciclovir ~90 mg/kg BID, topical 0.5% cidofovir), not steroids.[3]

Clinical facts

Feline herpesvirus-1 (FHV-1) is the classic cause of dendritic corneal ulceration, a branching epithelial lesion that is considered pathognomonic for the virus.[2] More than 80% of infected cats become persistently infected, establishing latency chiefly in the trigeminal ganglion, with recrudescence provoked by stressors — including systemic corticosteroid administration.[2]

The clinically important wrinkle from the 2025 report is that the pathogen in one dendritic-ulcer case was not a field strain but the attenuated F2 vaccine strain itself, recovered from a cat only 17 days after its first modified-live vaccination.[1] Vaccine strains are attenuated, not inert — under the right ocular conditions they can still replicate on the corneal surface.

This matters because the reflex to reach for a topical steroid in a "red, sore eye" is exactly wrong here: steroids both reactivate latent FHV-1 and, in a recently vaccinated cat, may unmask replication of the vaccine strain on an already-compromised cornea.[1][2]

Have a question about a specific feline keratitis case or a post-vaccination ocular presentation? Ask Voyage ForVets for an evidence-linked summary.

What the evidence shows

Anchor study — Suga & Kirisawa, JFMS 2025.[1] Four cats presented with dendritic corneal ulcers, and FHV-1 was isolated from all four. Whole-genome next-generation sequencing identified one isolate (the "NS strain") as the F2 modified-live vaccine strain, genomically identical to commercial F2 strains (Virbac and Intervet, and some Merial clones) and distinguished from field strains by a single-nucleotide variant in ORF28 and in ORF44. That cat (patient 3) had received the MLV F2 strain (Purevax RCP; Boehringer) as a first vaccination 17 days before the dendritic ulcer was diagnosed. The remaining three isolates were field strains. Notably, three of the four patients had a prior history of corticosteroid therapy. The authors conclude that the vaccine strain "has the potential to replicate in the corneal epithelium and form lesions," and that "in a cat with no previous history of allergy, corticosteroid administration is not recommended in the period immediately after MLV vaccination." As a single isolate this is a mechanistic caution — proof of concept that the vaccine strain can cause corneal disease — rather than an estimate of how often it does.

Supporting evidence — Gould, JFMS 2011.[2] This ocular-disease review establishes the framework the anchor finding slots into: dendritic ulcers are pathognomonic for FHV-1; >80% of exposed cats become persistently infected with trigeminal-ganglion latency; and latent virus is reactivated by stressors including systemic corticosteroids, with around 70% of latently infected cats shedding virus after corticosteroid administration. It further warns that corticosteroid use in chronic stromal keratitis "runs the very real risk of inducing viral recrudescence," and that topical corticosteroids in experimentally infected cats have induced corneal sequestrum formation. Together with the anchor, this gives two independent reasons to avoid steroids in a recently vaccinated cat with a herpetic ulcer: recrudescence of latent field virus and replication of the vaccine strain.

Therapeutic anchor — Thomasy & Maggs, Veterinary Ophthalmology 2016.[3] For the ulcerated (fluorescein-positive) eye, this review supports antiviral therapy as safe and effective: oral famciclovir at approximately 90 mg/kg twice daily achieved therapeutic penciclovir levels with reduced clinical signs and viral shedding, and topical 0.5% cidofovir twice daily reduced shedding and clinical disease in a masked, placebo-controlled study. (This review does not address corticosteroids; that caution rests on the anchor and Gould.)

How this fits clinical practice

• Treat the ulcer, not the inflammation. In any fluorescein-positive herpetic keratitis, withhold topical and systemic corticosteroids and use targeted antiviral therapy instead.[2][3]
• Take a recent-vaccination history. In a cat presenting with a dendritic ulcer within weeks of a first MLV FHV-1 vaccination, the vaccine strain is a plausible contributor — and an added reason to avoid steroids during that window.[1]
• Respect the latency biology. Even without recent vaccination, corticosteroids can reactivate latent FHV-1 in the majority of carriers, so a "steroid trial" for an unexplained feline red eye is a recognized trap.[2]
• Minimise stressors. Environmental stress reduction is part of managing recrudescent disease and complements antiviral therapy.[2]
• Scope of the signal. This is one documented vaccine-strain isolate, not a population risk estimate; it argues for caution and history-taking, not against MLV vaccination.[1]

For a case-specific evidence summary — including antiviral options and the steroid-timing question after modified-live vaccination — ask Voyage ForVets.

This brief is an evidence summary for licensed veterinary professionals and does not replace individual clinical assessment, current product labeling, or ophthalmology referral where indicated.

References

1. Suga Y, Kirisawa R. Isolation of the feline herpesvirus-1 modified live vaccine strain F2 from one of four cats with dendritic ulcers. Journal of Feline Medicine and Surgery 2025;27(1). https://pubmed.ncbi.nlm.nih.gov/39751391/
2. Gould D. Feline herpesvirus-1: ocular manifestations, diagnosis and treatment options. Journal of Feline Medicine and Surgery 2011;13(5):333-346. https://pmc.ncbi.nlm.nih.gov/articles/PMC11107991/
3. Thomasy SM, Maggs DJ. A review of antiviral drugs and other compounds with activity against feline herpesvirus type 1. Veterinary Ophthalmology 2016;19(Suppl 1):119-130. https://pmc.ncbi.nlm.nih.gov/articles/PMC4930706/

Changelog

• 2026-07-05: First published.